Tom239 writes: "In 2011, a federal court ordered the US TSA to publish and take comments on its "Advanced Imaging Technology" policy. A petition at whitehouse.gov asking the President to force the TSA to comply is already halfway to the threshold signature count that requires a response."

I think that people with a poor grasp of grammar and language rules don't recognize or assign as much weight to their absence.

This.

An otherwise competent writer may still not know when to use "which" vs. "that", why not to use a comma splice, or when precisely to use "whomever", and as a result may not see the value in following those grammatical rules. Someone who knows when to use an apostrophe, when to use "they're", "their" and "there", and when to use "John and me" correctly might consider themselves perfectly competent even without knowing the difference between an em-dash and an en-dash, or when to use a semicolon.

There arrives a point when one deserves a brand of linguistic competence but may not actually be perfect. Then there is the issue of style guides; When do I use numerals? When do I uppercase the first letter of words in titles? To serial-comma or not to serial-comma? Broken parallelism, anyone?